I do wonder what your SG was before fermentation, that could have told your approximate FG. I have back sweetened before freeze concentrating quite a few times, but I wanted the residual sugar to smooth out the alcohol "heat" during aging. I have used Splenda to back sweeten in the past; none of those batches were freeze concentrated so I don't know if Splenda will add a phony sweetener flavor or not. I would like to know how yours turns out and what the flavor is like after freeze concentrating.